Frequency divider and control method thereof

ABSTRACT

A frequency divider and a control method thereof are provided. The frequency divider includes a phase selector and a control circuit. The phase selector receives N input signals, and selects a first input signal from the N input signals as an output signal according to N selection signals. The frequency of the N input signals are the same, and the phase of the N input signals are different, and every adjacent two of the N input signals have a phase difference of 360°/N, wherein N is a positive integer larger than 2. The control circuit updates the N selection signals according to the output signal, such that the phase selector switches the output signal from the first input signal to a second input signal of the N input signals, wherein the phase of the second input signal leads the phase of the first input signal by 360°/N.

This application claims the benefit of Taiwan application Serial No.105101241, filed Jan. 15, 2016, the subject matter of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates in general to a frequency divider and a controlmethod thereof.

Description of the Related Art

The frequency divider of the prior art includes a phase selector whichswitches the output signal from an input signal to another input signal.However, the switching process can only be performed when the inputsignals are at low level or high level both before and after theswitching process. Otherwise, glitches will occur to the output signalof the frequency divider and cause error to the frequency generated bythe frequency divider. Thus, the frequency divider of the prior art canperform the switching process only during the period when the inputsignals have the same level both before and after the switching process(the phase difference between two input signals is defined as a period).According to one method provided for resolving the above problems, theinput signal with triangular wave is used, such that the input signalgradually switches to high level from low level, and the glitchesgenerated when the input signals have different levels will have asmaller amplitude. According to another method, the phase of the nextinput signal of the target input signal is detected, such that theswitching process is performed only when the phase of the target inputsignal and the phase of the current input signal have the same level.However, the above two methods cannot completely avoid the occurrence ofglitches. During the operation of the switching process, the frequencydivider is still subject to the condition that the two input signalsmust have the same level. Therefore, it has become a prominent task forthe industries to provide a method to avoid the occurrence of glitch.

SUMMARY OF THE INVENTION

According to an embodiment of the present disclosure, a frequencydivider is provided. The frequency divider includes a phase selector anda control circuit. The phase selector receives N input signals, andselects a first input signal from the N input signals as an outputsignal according to N selection signals. The frequency of the N inputsignals are the same, and the phase of the N input signals aredifferent, and every adjacent two of the N input signals have a phasedifference of 360° /N, wherein N is a positive integer larger than 2.The control circuit updates the N selection signals according to theoutput signal, such that the phase selector switches the output signalfrom the first input signal to a second input signal of the N inputsignals, wherein the phase of the second input signal leads the phase ofthe first input signal by 360°/N.

According to another embodiment of the present disclosure, a controlmethod of frequency divider is provided. The control method includesfollowing steps. A first input signal is selected from N input signalsas an output signal according to N selection signals, wherein thefrequency of the N input signals are the same, and the phase of the Ninput signals are different, every adjacent two of the N input signalshave a phase difference of 360°/N, and N is a positive integer largerthan 2. The N selection signals are updated according to the outputsignal, which is switched from the first input signal to a second inputsignal of the N input signals, wherein the phase of the second inputsignal leads the phase of the first input signal by 360°/N.

The above and other aspects of the invention will become betterunderstood with regard to the following detailed description of thepreferred but non-limiting embodiment (s). The following description ismade with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a frequency divider according to anembodiment of the present disclosure.

FIG. 2 is a timing diagram of a glitch generated when a conventionalfrequency divider switches between four input signals.

FIG. 3A is a timing diagram of the phase selector of FIG. 1 performing aswitching process during a first period according to an embodiment ofthe present disclosure.

FIG. 3B is a timing diagram of the phase selector of FIG. 1 performing aswitching process during a second period according to an embodiment ofthe present disclosure.

FIG. 3C is a timing diagram of the phase selector of FIG. 1 performing aswitching process during a third period according to an embodiment ofthe present disclosure.

FIG. 3D is a timing diagram of the phase selector of FIG. 1 performing aswitching process during a fourth period according to an embodiment ofthe present disclosure.

FIG. 4 is a circuit diagram of a control circuit according to anembodiment of the present disclosure.

FIG. 5 is a block diagram of a frequency divider according to anotherembodiment of the present disclosure.

FIG. 6 is a flowchart of a control method of frequency divider accordingto an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of a frequency divider 100 according to anembodiment of the present disclosure. The frequency divider 100 includesa phase selector 110 and a control circuit 130. The phase selector 110receives N input signals S1-SN, and selects a first input signal from Ninput signals as an output signal So according to N selection signalsC1-CN, wherein the N input signals have different phases, every adjacenttwo of the N input signals have a phase difference of 360°/N, and N is apositive integer larger than 2. The control circuit 130 updates the Nselection signals C1-CN according to an output signal So, such that thephase selector 110 switches the output signal from the first inputsignal to a second input signal of the N input signals, wherein thephase of the second input signal leads the phase of the first inputsignal by 360°/N.

The frequency divider of the present disclosure is exemplified by a4-phase frequency divider below. However, the present disclosure is notlimited thereto, and can also be used in a multi-phase frequencydivider. FIG. 2 is a timing diagram of a glitch generated when aconventional frequency divider switches between four input signals. Asindicated in FIG. 2, every adjacent two of the four input signals P1(0°), P2 (90°), P3 (180°), P4 (270°) have a phase difference of 90°. Attime point t0, the phase selector selects the input signal P1 (0°) as anoutput signal Po. At time point t1, the phase selector switches theoutput signal Po from the input signal P1 (0°) to the input signal P2(90°). Therefore, at time point t1 the level of the output signal Pocorrespondingly changes from high level to low level when the phaseselector switches the output signal Po from the input signal P1 (0°) tothe input signal P2 (90°). At time point t2, the level of the outputsignal Po corresponds to that of the input signal P2 (90°) and changesto high level. Under such circumstances, the output signal Po generatesa glitch designated by g1 as indicated in FIG. 2A. To avoid theoccurrence of glitch, the frequency divider of the prior art mustperform the switching process when the two input signals P1 (0°) and P2(90°) are both at high level, that is, during the switching period SW1between time point t3 and time point t4. Or, the frequency divider ofthe prior art must perform the switching process when the two inputsignals P1 (0°) and P2 (90°) both are at low level, that is, during theswitching period SW2 between time point t5 and time point t6. Under suchcircumstances, the frequency divider of the prior art can perform theswitching process only during the switching period SW1 and the switchingperiod W2. That is, the frequency divider of the prior art can performthe switching process during the one period (the phase differencebetween two input signals is defined as 1 period) when the input signalP1 (0°) is at high level or at low level.

In comparison to the frequency divider of the prior art, the frequencydivider 100 of the present disclosure can perform the switching processat any time and also avoid the occurrence of glitch. Referring to FIG.3A, a timing diagram of the phase selector 110 of FIG. 1 performing aswitching process during a first period according to an embodiment ofthe present disclosure is shown. As indicated in FIG. 3A, every adjacenttwo of the four input signals S1 (0°), S2 (270°), S3 (180°), S4 (90°)have a phase difference of 90°. In comparison to the prior art, duringthe switching process, the phase selector 110 of the present disclosureswitches the output signal to the input signal S2 (270°) from the inputsignal S1 (0°), wherein the phase of the input signal S2 (270°) leadsthe phase of the input signal S1 (0°) by a phase difference of 90°. Toput it in greater details, at time point t0′, the phase selector 110selects the input signal S1 (0°) as the output signal So. At time pointt1′, the phase selector 110 switches the output signal So to the inputsignal S2 (270°) from the input signal S1 (0°). Therefore, at time pointt1′, the level of the output signal So remains at high level when theoutput signal So is switched to the input signal S2 (270°) from theinput signal S1 (0°). Then, at time point t2′, the level of the outputsignal So corresponds to that of the input signal S2 (270°) and changesto low level. Therefore, no glitches are generated when the phaseselector 110 performs the switching process in the first period when thetwo input signals S1 (0°) and S2 (270°) both are at high level.

Referring to FIG. 3B, a timing diagram of the phase selector 110 of FIG.1 performing a switching process during a second period according to anembodiment of the present disclosure is shown. FIG. 3B is different fromFIG. 3A in that: the phase selector 110 of FIG. 3B performs theswitching process in the second period when the input signal S1 (0°) isat high level but the input signal S2 (270°) is at low level. To put itin greater details, at time point t3′, the phase selector 110 switchesthe output signal So to the input signal S2 (270°) from the input signalS1 (0°). Therefore, at time point t3′, the level of the output signal Sochanges to low level from high level when the output signal is switchedto the input signal S2 (270°) from the input signal S1 (0°). Then, attime point t4′, the level of the output signal So corresponds to that ofthe input signal S2 (270°) and changes to high level. Under suchcircumstances, no glitches are generated although the two input signalsS1 (0°) and S2 (270°) have different levels.

Referring to FIG. 3C, a timing diagram of the phase selector 110 of FIG.1 performing a switching process during a third period according to anembodiment of the present disclosure is shown. As indicated in FIG. 3C,the phase selector 110 performs the switching process in the thirdperiod when the input signal S1 (0°) and the input signal S2 (270°) bothare at low level. At time point t5′, the phase selector 110 switches theoutput signal So to the input signal S2 (270°) from the input signal S1(0°). Therefore, the level of the output signal So remains at low levelwhen the output signal So is switched to the input signal S2 (270°) formthe input signal S1 (0°). Then, at time point t6′, the level of theoutput signal So corresponds to that of the input signal S2 (270°) andchanges to high level. Therefore, no glitches are generated when thephase selector 110 performs the switching process in the third periodwhen the two input signals S1 (0°) and S2 (270°) both are at high level.

Referring to FIG. 3D, a timing diagram of the phase selector 110 of FIG.1 performing a switching process during a fourth period according to anembodiment of the present disclosure is shown. As indicated in FIG. 3D,the phase selector 110 performs the switching process in the fourthperiod when the input signal S1 (0°) is at low level but the inputsignal S2 (270°) is at high level. To put it in greater details, at timepoint t7′, the phase selector 110 switches the output signal So to theinput signal S2 (270°) from the input signal S1 (0°). Therefore, thelevel of the output signal So changes to high level from low level whenthe output signal is switched to the input signal S2 (270°) from theinput signal S1 (0°). Then, at time point t8′, the level of the outputsignal So corresponds to that of the input signal S2 (270°) and changesto low level. Under such circumstances, no glitches are generatedalthough the two input signals S1 (0°) and S2 (270°) have differentlevels.

As disclosed above, the phase selector 110 of the present disclosure canswitch the output signal to the input signal S2 (270°) from the inputsignal S1 (0°) from the first period to the fourth period withoutgenerating any glitches. That is, the phase selector 110 of the presentdisclosure can perform the switching process at any time withoutgenerating any glitches and is not subject to the condition that the twoinput signals S1 (0°) and S2 (270°) must be at the same level.

However, the present disclosure is not limited thereto. After the outputsignal is switched to the input signal S2 (270°) from the input signalS1 (0°), the phase selector 110 can also switch the output signal to theinput signal S3 (180°) from the input signal S2 (270°) withoutgenerating any glitches. Moreover, the present disclosure is not onlyapplicable to a 4-phase frequency divider, and is also applicable to amulti-phase frequency divider. That is, the phase selector 110 of thepresent disclosure can switch the output signal from an input signal toanother input signal whose phase leads the phase of the original inputsignal without generating any glitches.

FIG. 4 is a circuit diagram of a control circuit 130 according to anembodiment of the present disclosure. As indicated in FIG. 4, thecontrol circuit 130 includes four D-type flip-flops and an AND gate 120.The AND gate 120 generates a trigger signal Tr according to an outputsignal So and a control signal Sc, and updates the N selection signalsC1-C4 when the trigger signal Tr is enabled. In the present embodiment,it is exemplified that the 4-phase frequency divider is controlled usingfour D-type flip-flops. However, the present disclosure is not limitedthereto. For example, the control circuit 130 may include N D-typeflip-flops for controlling the N-phase frequency divider. The fourD-type flip-flops D1-D4 are connected in series from the first D-typeflip-flop D1 to the fourth D-type flip-flops D4. To put it in greaterdetails, each D-type flip-flop has an input end D, an output end Q and aclock input end C. The output end Q of the first D-type flip-flop 01 iscoupled to the input end D of the second D-type flip-flop D2. The outputend Q of the second D-type flip-flop D2 is coupled to the input end D ofthe third D-type flip-flop D3. By the same analogy, the output end ofthe last D-type flip-flop (the D-type flip-flop is 04 as exemplified inthe present example) is coupled to the input end D of the first D-typeflip-flop D1. The four D-type flip-flops D1-D4 all receive the triggersignal Tr. The output ends Q of the four D-type flip-flops D1-D4 providefour selection signals D1-D4 corresponding to 4 input signals S1 (0°),S2 (270°), S3 (180°), S4 (90°) respectively.

In the present example, at a specific time point, only the output end Qof one of the four D-type flip-flops is at high level. That is, only oneof the four selection signals D1-D4 is at high level. Meanwhile, thephase selector 110 selects the input signal S1 (0°) corresponding to thehigh-level selection signal as the output signal So.

To put it in greater details, suppose at the beginning the output end Qof the first D-type flip-flop is at high level, and the output ends Q ofthe remaining three D-type flip-flops D2-D4 are at low level. That is,the levels of the selection signals D1-D4 respectively are (1, 0, 0, 0).Meanwhile, the phase selector 110 selects the input signal S1 (0°)corresponding to the high-level selection signal as an output signal So.During the switching process, the control signal Sc is enabled andchanges to high level, the AND gate 120 receives the high-level controlsignal Sc and the output signal So, and generates an enabled high-leveltrigger signal Tr when both the control signal Sc and the output signalSo are at high-level. When the trigger signal Tr is enabled and changesto high level, the trigger signal Tr will trigger the four D-typeflip-flops. Meanwhile, the D-type flip-flop D1 is triggered and changesto low level because the D-type flip-flop D1 is coupled to the outputend Q of the D-type flip-flop D4. The D-type flip-flop D2 is triggeredand changes to high level from low level because the D-type flip-flop D2is coupled to the output end Q of the D-type flip-flop D1. The D-typeflip-flop D3 and the D-type flip-flop are also triggered but remain atlow level. That is, the outputs of the four D-type flip-flops D1-D4 willbe shifted to the right by one. That is, the levels of the selectionsignals D1-D4 change to (0, 1, 0, 0). Thus, the phase selector 110selects the input signal S2 (270°) corresponding to the high-levelselection signal as the output signal So.

Similarly, during the next switching process, the control signal Sc isenabled and changes to high level to trigger the four D-type flip-flops,and the outputs of the four D-type flip-flops D1-D4 will be shifted tothe right by one. That is, the levels of the selection signals D1-D4change to (0, 0, 1, 0). Thus, the phase selector 110 switches the outputsignal So to the input signal S3 (180°) from the input signal S2 (270°),and the rest can be obtained by analogy. Therefore, the control circuit130 of FIG. 4 enables the phase selector 110 to switch the output signalfrom an input signal to another input signal whose phase leads the phaseof the original input signal without generating any glitches. Thecontrol circuit 130 of FIG. 4 can be realized by four D-type flip-flops.However, the present disclosure is not limited thereto. Anyone ordinaryskilled in the technology will be able to make appropriate variationdesign to fit actual needs, and any variation design will do as long asfour selection signals can be provided to enable the phase selector 110to switch the output signal from an input signal to another input signalwhose phase leads the phase of the original input signal.

Moreover, the frequency divider 100 of the present disclosure is anN/(N-1) frequency divider. When the trigger signal Tr is not enabled andat low level, the frequency divider is in an N division mode, and istherefore divided by N. When the trigger signal Tr is enabled andchanges to high level, the output signal is switched forward by onephase, that is, the output signal is reduced by one phase, the frequencydivider is in an (N-1) division mode, and is therefore divided by (N-1).

FIG. 5 is a block diagram of a frequency divider 200 according toanother embodiment of the present disclosure. The frequency divider 200of FIG. 5 is different from the frequency divider 100 of FIG. 1 in that:the frequency divider 200 may include a dividing circuit 210. Thedividing circuit 210 generates N input signals S1-SN according to asignal Sin. The dividing circuit 210, realized by multiple flip-flops,divides the signal Sin by N to generate N input signals S1-SN having thesame frequency. Every adjacent two of the N input signals S1-SN have aphase difference of 360°/N. Suppose the frequency of the input signalSin is fin. Then, the frequency of the N input signals S1-SN is fin/N ofthe frequency of the input signal Sin.

The frequency divider 200 further includes a frequency divider circuit220, which is coupled between the phase selector 110 and the AND gate120. The frequency divider circuit 220 generates a division signal foaccording to the output signal So. The frequency divider circuit 220 canbe realized by a frequency divider, which is divided by k, wherein k isa positive integer larger than 1. When the control signal Sc is at lowlevel, the frequency of the division signal to of the frequency divider200 is: fin/(N×k). When the control signal Sc is at low level, thefrequency of the division signal to of the frequency divider 200 is:fin/[(N-1)×k]. Therefore, the frequency divider 200 can form multiplefrequencies according to the two modes.

A control method of frequency divider is provided in above embodimentsof the present disclosure. FIG. 6 is a flowchart of a control method offrequency divider according to an embodiment of the present disclosure.The frequency divider can be realized by such as the frequency divider100 of FIG. 1 or the frequency divider 200 of FIG. 5. The control methodincludes following steps. Firstly, the method begins at step S610, afirst input signal is selected from the N input signals as an outputsignal according to N selection signals, wherein the N input signalshave the same frequency but different phases, every adjacent two of theN input signals have a phase difference of 360°/N, and N is a positiveinteger larger than 2. Then, the method proceeds to step S620, a triggersignal is generated according to the output signal and a control signal.Then, the method proceeds to step S630, the N selection signals isupdated according to the output signal. Lastly, the method proceeds toS640, when the trigger signal is enabled, the N selection signals isupdated such that the output signal is switched from the first inputsignal to a second input signal whose phase leads the phase of the firstinput signal by 360°/N.

According to the frequency dividers and the control method thereofprovided in above embodiments, the frequency divider switches the outputsignal from an input signal to another input signal whose phase leadsthe phase of the original input signal without generating any glitches.Therefore, during the switching process, the frequency divider of thepresent disclosure can perform the switching process at any time pointand is not subject to the levels of the input signals.

While the invention has been described by way of example and in terms ofthe preferred embodiment (s), it is to be understood that the inventionis not limited thereto. On the contrary, it is intended to cover variousmodifications and similar arrangements and procedures, and the scope ofthe appended claims therefore should be accorded the broadestinterpretation so as to encompass all such modifications and similararrangements and procedures.

What is claimed is:
 1. A frequency divider, comprising: a phase selectorfor receiving N input signals, and selecting a first input signal from Ninput signals as an output signal according to N selection signals,wherein the frequency of the N input signals are the same, and the phaseof the N input signals are different, every adjacent two of the N inputsignals have a phase difference of 360°/N, and N is a positive integerlarger than 2; and a control circuit for updating the N selectionsignals according to the output signal, such that the phase selectorswitches the output signal from the first input signal to a second inputsignal of the N input signals, wherein the phase of the second inputsignal leads the phase of the first input signal by 360°/N.
 2. Thefrequency divider according to claim 1, wherein the control circuitcomprises: N flip-flops connected in series from a first flip-flop to anN-th flip-flop, wherein each of the N flip-flops has a clock input end,which receives a trigger signal, and output ends of the N flip-flopsprovide the N selection signals respectively; and an AND gate forgenerating the trigger signal according to the output signal and acontrol signal and further updating the N selection signals when thetrigger signal is enabled.
 3. The frequency divider according to claim2, wherein each of the N flip-flops has an input end and the output end,the output end of an i-th flip-flop is coupled to the input end of an(i+1)-th flip-flop, the output end of the N-th flip-flop is coupled tothe input end of the first flip-flop, i is a positive integer and1≦i≦N-1, the output end of the i-th flip-flop provides the selectionsignal corresponding to the first input signal, and the output end ofthe (i+1)-th flip-flop provides the selection signal corresponding tothe second input signal.
 4. The frequency divider according to claim 3,wherein the phase selector selects the first input signal as the outputsignal when the output end of the i-th flip-flop is at high level, andthe phase selector selects the second input signal as the output signalwhen the output end of the (i+1)-th flip-flop is at high level.
 5. Thefrequency divider according to claim 1, wherein the frequency divider isan N/(N-1) frequency divider; when the trigger signal is enabled, thefrequency divider is in an (N-1) division mode; when the trigger signalis not enabled, the frequency divider is in an N division mode.
 6. Thefrequency divider according to claim 1, wherein the frequency dividerfurther comprises: a frequency divider circuit coupled between the phaseselector and an AND gate for generating a division signal according tothe output signal.
 7. The frequency divider according to claim 1,wherein the frequency divider further comprises: a dividing circuit forgenerating the N input signals according to a signal.
 8. A controlmethod of frequency divider, comprising: selecting a first input signalfrom N input signals as an output signal according to N selectionsignals, wherein the frequency of the N input signals are, and the phaseof the N input signals are different, every adjacent two of the N inputsignals have a phase difference of 360°/N, and N is a positive integerlarger than 2; and updating the N selection signals according to theoutput signal to switch the output signal from the first input signal toa second input signal of the N input signals, wherein the phase of thesecond input signal leads the phase of the first input signal by 360°/N.9. The control method according to claim 8, further comprising:providing the N selection signals using N flip-flops respectively; andgenerating a trigger signal according to the output signal and a controlsignal, and updating the N selection signals when the trigger signal isenabled; wherein the N flip-flops are connected in series from a firstflip-flop to an N-th flip-flop, each of the N flip-flops has a clockinput end, which receives the trigger signal, and output ends of the Nflip-flops provide the N selection signals respectively.
 10. The controlmethod according to claim 9, wherein each of the N flip-flops has aninput end and the output end, the output end of an i-th flip-flop iscoupled to the input end of an (i+1)-th flip-flop, the output end of theN-th flip-flop is coupled to the input end of the first flip-flop, i isa positive integer and 1≦i≦N-1, the output end of the i-th flip-flopprovides the selection signal corresponding to the first input signal,and the output end of the (i+1)-th flip-flop provides the selectionsignal corresponding to the second input signal.
 11. The control methodaccording to claim 19, further comprising: selecting the first inputsignal as the output signal when the output end of the i-th flip-flop isat high level; and selecting the second input signal as the outputsignal when the output end of the (i+1)-th flip-flop is at high level12. The control method according to claim 8, wherein the frequencydivider is an N/(N-1) frequency divider; when the trigger signal isenabled, the frequency divider is in an (N-1) division mode; when thetrigger signal is not enabled, the frequency divider is in an N divisionmode.
 13. The control method according to claim 8, further comprising:generating a division signal according to the output signal using afrequency divider circuit.
 14. The control method according to claim 8,further comprising: generating the N input signals according to a signalusing a dividing circuit.